1. Field of the Invention
This invention relates to novel, composite agglomerated limestone particles useful for absorbing sulfur oxides (SO.sub.2 and SO.sub.3) or hydrogen sulfide from hot gases arising from combustion, gasification or other chemical reaction of coal or other organic fuels. More particularly, this invention relates to a method of making such agglomerates such that resistance to attrition is equivalent to conventionally used natural limestone granules.
2. Description of the Prior Art
A wide variety of methods are currently employed to remove sulfur compounds from gaseous streams. Hydrogen sulfide, arising for example from coal gasification operations, is removed to purify reducing gases for other uses downstream, such as further chemical reaction. Sulfur oxides and hydrogen sulfides when present must be removed from exhaust gases to maintain environmental air quality. Sulfur oxides arise from burning carbonaceous, sulfur-bearing fuels such as coal or heavy oil. It has long been known that the calcium in limestone and the calcium and magnesium in dolomite are effective sulfur capturing agents. The stone may be pre-calcined to convert the calcium or magnesium carbonate to the oxide, which reacts with sulfur compounds to produce in the first instance, calcium sulfide from hydrogen sulfide gas, or in the second instance, calcium sulfate from sulfur oxides and oxygen. High temperature is generally required for the reactions in the dry state and the calcination reaction (release of CO.sub.2) may take place simultaneously with the absorption reaction. Wet scrubbing methods may be used effectively to remove sulfur compounds in gases at lower temperature.
Limestone sorbents for sulfur oxide find particular application in the fluidized bed combustor, which is a promising technology for more efficient industrial steam generation or electrical power generation. The limestone is used in the form of crushed granules, irregular in shape and about 1/4 inch in size. The limestone granules are mixed directly in the fluidized bed with pulverized coal of similar shape and size or a heavy oil spray. An upward flow of air supplies oxygen for burning the fuel and fluidizing the bed. Sulfur contained in the fuel is oxidized and subsequently absorbed by the limestone in a high temperature, dry-state reaction. Spent sorbent and ash are removed from the bed continuously as fresh limestone and fuel are added. Reference is made to the following two articles explaining the state of the art in this technology: C. S. R. Rao, "FLUIDIZED BED COMBUSTION TECHNOLOGY--A Review," Combustion Science and Technology, vol. 16 215-227 (1977), and J. E. Mesko, "COAL COMBUSTION IN A LIMESTONE BED," Chemical Engineering Progress, vol. 74, no. 8, pages 99-102 (1978).
One of the major problems associated with the operation of these fluidized bed combustors (FBC's) is the excess limestone required to meet current EPA air pollution standards for sulfur oxide emissions. Depending upon the reactivity of the limestone, at least three times the stoichiometric amount (Ca/S ratio of 1) may be required for 85% SO.sub.2 removal. Reference is made to EPRI Final Report, Project 721-1 "CRITERIA FOR THE SELECTION OF SO.sub.2 SORBENTS FOR ATMOSPHERIC PRESSURE FLUIDIZED-BED COMBUSTORS," EPRI FP-1307, Vol. 1, December, 1979, published by Westinghouse Electric Corp., Pittsburgh, Pa. Proposed stricter standards for sulfur oxide (SO.sub.x) levels will lead to even higher Ca/S ratios. This large sorbent requirement has a significant effect on FBC-boiler plant cost and performance. The spent sorbent generally contains only a relatively small portion (about 30% or less) of the total available calcium converted to calcium sulfate. The remaining calcium exists mostly as calcium oxide with small amounts of calcium carbonate. The loss in reactivity is attributed to calcium sulfate product plugging the pores of the calcined limestone so that further reaction cannot continue. Thus the majority of potential sorbent value remains unused. For these reasons enhanced reactivity and improved calcium utilization would be highly desirable for reducing the Ca/S ratio.
Several methods for enhancing the reactivity of limestone granules have been investigated in the past. Reference is made to L. L. Gasner and S. E. Setesak, "LIMESTONE UTILIZATION OPTIMIZATION IN FLUIDIZED BED BOILERS," Proceedings of the Fifth International Conference on Fluidized Bed Combustion, Vol. II, Mitre Corp., 1978 (pages 762-772). The conference at which this report was originally presented was held in December 1977 in Washington, D. C. These authors discuss three favored methods of reactivity enhancement; salt (sodium chloride) addition, slow precalcination in an atmosphere rich in carbon dioxide, and reduction of the limestone to particles finer than -325 mesh Tyler. All three methods have serious drawbacks, and the authors conclude the best way of increasing reactivity is to agglomerate finely powdered limestone. Gasner et al, supra, propose a process in which spent limestone agglomerated with fly ash would be used as a recycle with unreformed limestone granules as fresh feed. A classification step would separate the two after their time in the reactor. It is not known that agglomerates so formed could withstand the conditions of combustion in a fluidized bed, or whether fly ash is used as a binder or sorbent. In a general desulfurization application U.S. Pat. No. 4,061,716 to McGauley makes fleeting reference to an agglomerated calcium-bearing sorbent such as limestone, lime or dolomite, bound with a water-soluble and reversibly hydratable compound such as calcium sulfate or calcium hydroxide. The patent contains no examples illustrating the production of calcium-based sorbents, and there is no indication that sorbents disclosed by McGauley, supra would possess the required heat stability or would give the required resistance to attrition for fluidized bed use.
Other examples of limestone agglomerates for desulfurization of gases are disclosed by Netherlands application No. 76/03,614 published Oct. 11, 1977 on application No. 76/3,614 published Apr. 7, 1976, assigned to Firma Hermann Wegner (CHEMICAL ABSTRACTS 89; 48340a). Limestone is first burned (heated to 400.degree. C.) before being impregnated with a binder such as sodium silicate or sodium borate in solution. The mixture is then pelletized. German Offenlegung No. 2,548,845 to Dolmon et al published May 13, 1976 with a French application No. 74/36,862 published Nov. 6, 1974 (CHEMICAL ABSTRACTS 85; 129864u) discloses the use of halide salts such as calcium chloride, potassium chloride or calcium bromide to bind calcium carbonate powders, and sulfuric acid to bind magnesium compounds (oxide, hydroxide, carbonate) to form pelletized sorbents for desulfurization of gases.
Another example of agglomerated calciferous material is one suggested for absorbing sulfur dioxide from automobile exhaust gas disclosed in U.S. Pat. No. 4,061,593 to Summers. Calcium hydroxide is pelletized with sodium silicate as a binder. The mixture is pelletized in a Dorst compacting press. The cylindrically-shaped pellet is then partially calcined in a CO.sub.2 environment to convert part of the CaO to CaCO.sub.3. The carbonate and silicate form a rigid matrix within the particle so that the particle does not swell appreciably when reacting with sulfur dioxide and has sufficient crush strength to allow transportation without deterioration.
Still other examples of agglomerated limestone particles containing binders have been suggested for use in agriculture and ceramics. Reference is made to Perrine (U.S. Pat. No. 4,015,973) which discloses the use of a swelling, i.e., sodium, bentonite as a binder for crushed limestone to produce pelletized granules useful primarily as soil conditioners. Binder is present in amounts ranging from 1 to 10% based on total weight, and water is added during the pelletizing step. A pan agglomerator is preferred, since more uniform pellets allegedly result from its use over mixer or drum agglomerators. U.S. Pat. No. 3,615,811 to Barrett discloses alkali metal silicates, bentonite, and various organic compounds as binders for alkaline earth carbonate particles useful in ceramics. Binder is present in amounts up to 5% based on total weight. The particles are formed by slurrying the ingredients and spray drying.
Like the bentonites, attapulgite clay is widely known and used as a binder for various materials. A zeolite agglomerate bound with 3-6% attapulgite clay based on total weight of the particle is disclosed in U.S. Pat. No. 3,287,281 to Haden et al. U.S. Pat. No. 2,831,818 discloses bone meal bound with about 20% attapulgite and 6% starch (dry weight basis). U.S. Pat. No. 3,098,045 to Allegrini et al discloses bauxite aggregates bound with up to about 15% attapulgite clay.